1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of the Related Art
In recent years, demands for energy-saving and speed-enhancement technologies have been rising in the market for image forming apparatuses such as printers, copiers, and facsimiles.
In image forming apparatuses, an unfixed toner image is formed in an image transfer method or in a direct method on recording materials such as recoding sheets, printing paper, photosensitive paper, and electrostatic recording paper by the image forming process such as electrophotographic recording, electrostatic recording, and magnetic recording. As a fixing device to fix an unfixed toner image, a fixing device based on a contact heating method such as a hot holler method, a film heating method, and an electromagnetic induction heating method is widely used.
Challenges in such a fixing device in recent years include the following.
To further reduce in warmup time that is time required until a predetermined temperature (reload temperature) that enables printing is reached from room temperature at the time of power activation, or first print time that is time requited until printing operation is performed through printing preparation process, and ejection of paper is completed from reception of a print request (Challenge 1).
To solve the problem of so-called temperature drop in which amount of heat becomes insufficient especially at the beginning of continuous printing because with enhanced speed of image forming apparatuses, the sheets of paper fed thereinto per unit time increases and a necessary amount of heat increases (Challenge 2).
To solve the above problems, the applicant of the present application has proposed a fixing device that is configured such that an endless belt having a low heat capacity is directly heated (not through a metallic heat conductor), and that enables to achieve favorable fixability even if equipped in highly productive image forming apparatuses.
However, with a fixing configuration using an endless belt having a low heat capacity, it is difficult to keep the temperature distribution uniform in the longitudinal direction at the time of feeding paper. That is, although heat is consumed in a region in which a recording medium in a small size passes (paper passing region) to heat up the recording medium (a recording sheet and unfixed toner on the recording medium), heat is not lost by a recording medium in a non-paper-passing region. Therefore, heat is accumulated in a heating roller and a belt, and so-called end temperature rise is known to occur in which temperature at a nip portion in this non-paper-passing region becomes higher than temperature at a nip portion in the paper passing region at which temperature is managed to be maintained at predetermined temperature.
To tackle this problem of the end temperature rise, one that is intended to prevent end temperature rise by using a heat-equalization material in a part of a nip forming member that is pressurized to be in contact with a fixing belt to enhance performance in thermal transfer and heat absorption has been proposed.
In Japanese Patent Laid-open Publication No. 2004-235001 (Patent Literature 1), a fixing device in which a fixing member is formed with a heat-resistant film, and a heat-equalization member is arranged on a downstream side of a fixing nip portion is disclosed.
However, for a configuration using a heat-equalization material, there has been a problem in which temperature at an end portion becomes low compared to a central portion at the time of activation of fixing, as tradeoff for preventing occurrence of the end temperature rise.
Therefore, it is desirable to provide a fixing device that enables to prevent occurrence of end temperature rise and to suppress reduction in heat at an end portion at the time of activation.